Aluminum master alloys



United States Patent US. Cl. 75-135 9 Claims ABSTRACT OF THE DISCLOSUREAluminum master alloys, containing finely divided transition metalboride dispersed throughout the aluminum, are prepared by forming amolten mixture of aluminum, a transition metal boride and a flux andcooling the molten mixture. The flux is an inorganic fluoride whichassists in the wetting of the boride with the aluminum.

This invention relates to aluminum alloys containing a finely dividedmetal boride, and tothe preparation of said alloys useful in theproduction of fine grain aluminum.

It is known that the presence of up to about 100 p.p.m. by weight of ametallic boride having a hexagonal structure, such as the transitionmetal borides, confers a fine grain structure on cast aluminum, which isof great importance when the aluminum is to be rolled into sheet orfoil. In general, these borides should be used in the form of finepowders as their effectiveness depends upon the number of particlespresent; and moreover, large particles are abrasive and lead toinhomogeneity.

In practice, it is virtually impossible to add the requisite very smallquantity of a finely divided boride to molten aluminum without theformer being oxidized during the addition, and rendered inelfective as agrain refining agent. To overcome this difliculty it is customary toprepare a master alloy of aluminum containing boron, and titanium oranother metal which will form a hexagonal boride, said alloy being inthe form of the elements, or the boride, or a mixture of the two, and touse this alloy as a source of boride in the preparation of fine grainaluminum. Such master alloys contain for instance, from 0.5 to about 10%or more, by weight of boride. Hitherto, these master alloys have beenmade by the reaction of aluminum with a mixture of complex fluorides ofthe metal from which the boride is derived and of boron. An example ofone such process is:

However, this process is expensive and relatively inefficient, and theproduct contains an undesirably high proportion of coarse particles.

We have now found that finely divided borides, such as of particle sizeless than about microns, can be evenly dispersed in aluminum to form amaster alloy by means of the use of a suitable flux which causes theboride to be thoroughly wetted by the aluminum. The use of the flux alsoserves to prevent the oxidation of the finely divided particles ofboride.

Accordingly, the present invention comprises in one aspect a process forthe preparation of a master aluminum alloy which comprises forming amolten mixture of aluminum, a finely divided metal boride, and a flux,said flux comprising a fluoride which assists in the wetting of theboride with the aluminum. Metal borides which can be used are thetransition metal borides, such as titanium diboride, chromium diboride,zirconium diboride and vanadium diboride, and preferably those capableof existing in a hexagonal form.

3,464,816 Patented Sept. 2, 1969 ice The fluxes which have been found tobe particularly effective are complex halides, especially the inorganicfluorides such as K ZrF K TiF Na A-lF potassium fluoride, and mixturesof potassium fluoride with potassium chloride or with potassium iodideor with a mixture of the two. In general, any complex halide or mixtureof halides containing a fluoride as one component which causes theboride being used to become wetted by aluminum can be used. The amountof flux used is not narrowly critical, varying from as little as aboutone hundredth of the weight of boride to an amount equal to the weightof the boride. Preferably, at least about one tenth of the weight of theboride is used.

The aluminum master alloy can be made, for instance, by mixing thefinely divided boride with the flux, and blending the mixture withmolten aluminum. Alternatively, the mixture of the flux and boride canbe heated with solid aluminum until the metal melts, and then themixture is stirred. On cooling, a solid master alloy containing thefinely divided boride dispersed throughout is obtained. This in turn canbe added to larger quantities of molten aluminum in order to obtain afinal product containing, for example, up to about 100 p.p.m. of theboride in a form which results in the final cast aluminum product to befine grained.

The resultant alloy contains boride of a known and controlled particlesize evenly dispersed throughout the aluminum. Such alloys are farsuperior to previous master alloys which contained particles of widelyvarying sizes and of uncontrolled distribution.

The invention is illustrated by the following examples, but it is to beunderstood that my invention is not restricted to the specific examplesgiven.

EXAMPLE I Ten grams of titanium diboride having a mean particle size of2.5 microns was mixed with six grams of potassium hexafluorozirconateand placed in an alumina crucible. Ninety grams of aluminum in a singlepiece was placed on top of the mixture and the crucible was heated untilthe aluminum and the flux had melted. The mixture was stirred andallowed to cool. The solid product, obtained after being freed fromexcess flux by washing with water, contained 9.5% of finely dividedtitanium diboride evenly dispersed throughout the aluminum.

EXAMPLE II The procedure of Example I was repeated using 7.5 grams oftitanium diboride 2.5 grams of sodium aluminum fluoride (cryolite) and100 grams of aluminum. A master alloy containing at least of thetitanium diboride evenly dispersed in aluminum was obtained.

EXAMPLE III The procedure of Example I was repeated usnig 7.5 grams ofchromium diboride, 2.5 grams of a 1:1, by Weight, mixture of potassiumfluoride and potassium iodide, and grams of aluminum. A master alloycontaining at least 95% of the chromium diboride evenly dispersed inaluminum was obtained.

EXAMPLE IV The procedure of Example I was repeated using 7.5 grams ofzirconium diboride, 2.5 grams of sodium aluminum fluoride and 100 gramsof aluminum. The resultant master alloy contained at least 95% of thezirconium diboride evenly dispersed in the aluminum.

Similar results were obtained using 2.5 grams of potassium fluoride asflux instead of the sodium aluminum fluoride.

What is claimed is:

1. The process for the preparation of a master aluminum alloy whichcomprises forming a molten mixture of aluminum, a finely dividedtransition metal boride and a flux, and cooling said molten mixture toform a master aluminum alloy having said transition metal boridedispersed throughout said aluminum, said flux comprising an inorganicfluoride which assists in the wetting of said boride with said aluminum.

2. The process according to claim 1 wherein said finely divided borideand said flux are mixed and then blended with molten aluminum.

3. The process according to claim 1 wherein said aluminum, said borideand said flux are heated until the metal melts and the mixture isstirred to disperse the boride throughout the aluminum.

4. The process according to claim 1 wherein said boride has a hexagonalstructure.

5. The process according to claim 1 wherein said boride is titaniumdihoride.

6. The process according to claim 1 wherein said flux is potassiumhexafiuorozirconate.

References Cited UNITED STATES PATENTS 8/1933 Bonsack 75138 6/1962Conant 75-138 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75138

